(1) Field of Invention
The present invention relates to cementitious compositions, and more particularly refers to a gypsum composition having a higher degree of water-resistance than has been obtained in the prior art.
(2) Description of the Prior Art
Regular gypsum wallboard and similar products have very little resistance to water. When ordinary gypsum wallboard is immersed in water at room temperature, for example, 70.degree. F., the lack of water resistance is readily demonstrated by the fact that in a short time the gypsum core has absorbed a considerable amount of water. Actual tests have demonstrated that when a 2.times.2.times.2 inch cube of gypsum board core material is immersed in water at about 70.degree. F., the material shows a water absorption of greater than 60% after having been immersed for 5 minutes. Many attempts have been made in the past to improve the water resistance of gypsum products. These attempts included the incorporation of water-resistant materials such as metallic soaps, asphalts, waxes, resins, etc., within the wet calcium sulfate mixture. They have also included attempts to coat the finished gypsum board with water-resistant films or coatings.
One example of past attempts to waterproof gypsum products integrally by the addition of water-repellent substances is that disclosed in the King and Camp U.S. Pat. No. 2,198,776. This disclosure teaches the incorporation of paraffin wax, asphalt, or other fusible water-insoluble organic substances into a gypsum mix by first melting this waxy material and then dispersing such material into the wet gypsum mix by spraying the hot molten mass into the aqueous gypsum dispersion. This brings about a formation of finely dispersed solidified paraffin or asphalt particles which then serve to improve the water resistance of the gypsum product. While these past efforts resulted in some degree of success, there has been no waterproofing process developed in the past which was capable of causing a drastic increase in water-resistance by the use of only a comparatively very small amount of cheap waterproofing materials.
Camp U.S. Pat. No. 2,432,963 discloses that if a mixture of paraffin wax and asphalt, in definite proportions, say on the order of from about 5% to about 15% (dry basis), is incorporated in the form of an emulsion with a wet plastic gypsum mass, the finished dried gypsum wallboard will possess a degree of water resistance which is greatly out of proportion to the degree of water resistance obtained if one uses these same paraffin wax and asphalt materials witout emulsification. The degree of water-resistance obtained by the use of a mixture of paraffin wax and asphalt in certain proportions is very much greater than that obtained when one uses either the wax alone or the asphalt alone in emulsified form. Camp U.S. Pat. No. 2,526,537 has disclosed a water-resistant gypsum product having added therein a composition of asphalt, paraffin wax and potassium sulfate.
Camp U.S. Pat. No. 2,526,538 discloses a water-resistant product comprising gypsum and a composition of asphalt, paraffin wax and a substance selected from the group consisting of alkali and alkaline earth aluminates and Portland cement.
Selbe U.S. Pat. No. 2,669,414 discloses a water-resistant gypsum composition formed by adding to a gypsum slurry a blend of oil-soluble alkali salt of mahogany sulfonic acid with asphalt and paraffin wax.
In "The Elvasize Process" published by the du Pont Company there is described a method for first coating polyvinyl alcohol on paper and then adding borax to control the penetration of the polyvinyl alcohol size into the paper. However, the process described is for sizing paper and not for preparing water-resistant gypsum board.
U.S. Pat. No. 3,853,689 teaches that the sag resistance of gypsum board is improved by incorporating a small amount of polyvinyl alcohol in the gypsum core of the board.
U.S. Pat. No. 3,935,021 teaches that the water resistance of gypsum products such as gypsum wallboard is improved by incorporating into the composition from which the gypsum product is made a combination of polyvinyl alcohol and asphalt-wax emulsion. However, the patent teaches that at least 0.15 weight percent of polyvinyl alcohol must be utilized in order to obtain acceptable results. Since the cost of polyvinyl alcohol is substantial, economic considerations represent a formidable block to the commercialization of this patented product. In evaluating the prior art discussed above, it is clear that many of the compositions yield products having good water-resistant properties, but that the cost of the materials is undesirably high and that a less expensive means of rendering gypsum board water-resistant would be highly desirable to obtain a competitive advantage in the market. Further, the incorporation of abnormally high amounts of materials such as asphalt and wax to obtain an even higher degree of water resistance may have deleterious effects on the properties of the finished product.